An analysis of the methyl rotation dynamics in the So (x' A) and T (a A) states of thioacetaldehyde from Pyrolysis jet spectra /

dc.contributor.author

Bascal, Hafed Ashur.

en_US

dc.date.accessioned

2009-07-14T19:41:58Z

dc.date.available

2009-07-14T19:41:58Z

dc.date.issued

1991-07-14T19:41:58Z

dc.identifier.uri

http://hdl.handle.net/10464/2324

dc.description.abstract

Jet-cooled, laser-induced phosphorescence excitation
spectra (LIP) of thioacetaldehyde CH3CHS, CH3CDS, CD3CHS and
CD3CDS have been observed over the region 15800 - 17300 cm"^ in
a continuous pyrolysis jet. The vibronic band structure of
the singlet-triplet n -* n* transition were attributed to the
strong coupling of the methyl torsion and aldehydic hydrogen
wagging modes . The vibronic peaks have been assigned in terms
of two upper electronic state (T^) vibrations; the methyl
torsion mode v^g, and the aldehydic hydrogen wagging mode v^^.
The electronic origin O^a^ is unequivocally assigned as
follows: CH3CHS (16294.9 cm"'' ), CH3CDS (16360.9 cm"'' ), CD3CHS
(16299.7 cm"^ ), and CD3CDS (16367.2 cm"'' ).
To obtain structural and dynamical information about the
two electronic states, potential surfaces V(e,a) for the 6
(methyl torsion) and a (hydrogen wagging) motions were
generated by ab initio quantum mechanical calculations with a
6-3 IG* basis in which the structural parameters were fully
relaxed. The kinetic energy coefficients BQ(a,e) , B^(a,G) , and
the cross coupling term B^(a,e) , were accurately represented
as functions of the two active coordinates, a and 9. The
calculations reveal that the molecule adopts an eclipsed
conformation for the lower Sq electronic state (a=0°,e=0"')
with a barrier height to internal rotation of 541.5 cm"^ which
is to be compared to 549.8 cm"^ obtained from the microwave
experiment. The conformation of the upper T^ electronic state
was found to be staggered (a=24 . 68° ,e=-45. 66° ) . The saddle
point in the path traced out by the aldehyde wagging motion
was calculated to be 175 cm"^ above the equilibrium
configuration. The corresponding maxima in the path taken by
methyl torsion was found to be 322 cm'\ The small amplitude normal vibrational modes were also calculated to aid in the
assignment of the spectra. Torsional-wagging energy manifolds
for the two states were derived from the Hamiltonian H(a,e)
which was solved variationally using an extended two
dimensional Fourier expansion as a basis set. A torsionalinversion
band spectrum was derived from the calculated energy
levels and Franck-Condon factors, and was compared with the
experimental supersonic-jet spectra. Most of the anomalies
which were associated with the interpretation of the observed
spectrum could be accounted for by the band profiles derived
from ab initio SCF calculations.
A model describing the jet spectra was derived by scaling
the ab initio potential functions. The global least squares
fitting generates a triplet state potential which has a
minimum at (a=22.38° ,e=-41.08°) . The flatter potential in the
scaled model yielded excellent agreement between the observed
and calculated frequency intervals.

en_US

dc.language.iso

eng

en_US

dc.publisher

Brock University

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dc.subject

Thioacetaldehyde.

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dc.subject

Pyrolysis.

en_US

dc.subject

Spectrum analysis.

en_US

dc.subject

Methyl groups

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dc.subject

Rotation groups

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dc.subject

Aerodynamics, Supersonic.

en_US

dc.subject

Aliphatic compounds.

en_US

dc.title

An analysis of the methyl rotation dynamics in the So (x' A) and T (a A) states of thioacetaldehyde from Pyrolysis jet spectra /